The present invention relates to a strain detector for detecting the strain of a passive member.
FIG. 4 illustrates a conventional strain detector of the sort disclosed in Japanese Patent Unexamined Publication No. Sho. 59-164931, wherein there are shown an arrangement of a passive member 1 for receiving rotating torque, magnetostrictive layers 2 made of high permeability soft magnetic material and secured to the passive member 1 in the form of a belt, the belt-like magnetostrictive layers being arranged symmetrically about the central axis at angles of .+-.45.degree., and detection coils 3 provided around the magnetostrictive layers 2, respectively.
In operation, the stress maximized on the surface of the passive member 1 is produced when rotating torque is applied thereto from the outside and the principal axis of the principal stress is in the direction of the longitudinal axis of the magnetostrictive layer 2 formed of belt-like elementary strips. Assuming the principal stress is tensile force with respect to a group of elementary strips of the magnetostrictive layer 2 in one direction, it turns into compression force with respect to a group of elementary strips of the other magnetostrictive layer 2 perpendicular to that direction. When stress is applied to magnetic material whose magnetostriction constant is not zero, the permeability generally changes. When the magnetostriction constant as a quantity representing the amount of magnetostriction quantitatively is positive, the permeability will increase if tensile force acts on it, whereas it will decrease if compression force acts on it. When the magnetostriction constant is negative, the results are reversed. The detection coil 3 detects variations of the permeability in the magnetostrictive layer 2 in proportion to the quantity of torque applied from the outside as variations of magnetic impedance and also detects the quantity of torque applied to the passive member 1 and the quantity of strain accompanied therewith. As the outputs of the detection coils 3 are opposite in polarity, a large output is made available by obtaining the difference output.
Amorphous magnetic material may conventionally be used for such a magnetostrictive layer; however, it has a shortcoming in that its magnetic characteristics are liable to change by its temperature changes because its Curie temperature is relatively low and because its crystalline structure is unstable. In the case of the strain detector described above, a plating film of nickel, permalloy or iron is subjected to heat treatment before being used for the magnetostrictive layer 2. Therefore, the magnetostrictive layer 2, which is firmly secured to the passive member, has the following features: stability in view of crystalline structure, not only mechanical but also thermal stability and resulting high reliability.
The strain detector described above employs the plating film mainly composed of nickel for the magnetostrictive layers. However, the plating film tends to easily take in impurities during the plating process and particularly a trace of sulfur are introduced from an electrodeposition stress relaxation agent and the like. Although the plating film is heat-treated to increase strain detecting sensitivity, the heat treatment allows the sulfur of the impurities to combine with nickel to form a sulfide. Moreover, the sulfide is segrated at the crystal grain boundary, which makes the plating film brittle, and when torque is applied to the passive member, very small cracks are developed in the magnetostrictive layer, which is described in, for instance, Technical document, Technical Report of Kawasaki Steel Corporation, p-12, No. 4, Vol. 14 (1982). Thus, the problem arises that a hysteresis may occur in the output characteristics of the detection coil as shown in FIG. 5.